1. Field of the Invention
The present invention relates to a lighting circuit for supplying a voltage to a start circuit that generates a high voltage signal to a discharge lamp.
2. Description of the Related Art
For a lighting circuit of a discharge lamp such as a metal halide lamp or the like, use of a DC power source circuit, a DC-AC conversion circuit, and a start circuit (so-called starter circuit) is well known.
For example, a DC-DC converter is used in the DC power source circuit. A full bridge type circuit comprising two pairs of semiconductor switch elements or switching elements for conducting switch control is used as a DC-AC conversion circuit and a driver circuit for controlling the switching elements. After an output voltage of the DC-DC converter is converted into a rectangular wave-shaped voltage, this voltage is supplied to the discharge lamp.
If the lighting circuit is separately provided for each of a plurality of discharge lamps, the number of parts and costs become a concern. Therefore, common circuits may be used as the DC power source circuit and the DC-AC conversion circuit.
The voltage supplied to the start circuit is generated according to the output of the DC power source circuit or the DC-AC conversion circuit. However, if the voltage supply circuit for each start circuit is separately provided, the cost and the size of the apparatus may increase.
Accordingly, the present invention provides a lighting circuit for a plurality of discharge lamps where a common circuit for supplying a voltage to the start circuit is provided for each of the discharge lamps. Thus, reduction in the cost and the size of the apparatus can be achieved.
The present invention relates to a discharge lamp lighting circuit. A DC power source circuit is provided to output a DC voltage and a DC-AC conversion circuit converts the output voltage of the DC power source circuit into an AC voltage. The AC voltage is supplied to a discharge lamp. Start circuits provided for each of the discharge lamps to respectively generate a starting high voltage signal to a plurality of discharge lamps.
The present invention comprises the structures shown as (A) to (E):
(A) The DC power source circuit outputs a bipolar voltage of positive and negative polarity to the ground potential.
(B) The DC-AC conversion circuit is provided with a plurality of switch elements to switch the output voltage of each polarity from the DC power source circuit and to send the switched voltage to each discharge lamp. It also has a drive circuit to conduct the switching control of the elements.
(C) Each start circuit has a transformer including a magnetic substance and a primary coil and a secondary coil. A primary circuit of the transformer has a serial circuit of a capacitor and a switch element, and the serial circuit is connected in parallel to the primary coil of the transformer.
(D) When the voltage across both terminals of the capacitor exceeds a threshold value according to the charge accumulation of the capacitor in the primary circuit of the transformer, or when the switch element of (C) achieves electrical continuity, the starting high voltage signal is applied to the discharge lamp from the primary coil through the secondary coil.
(E) A common voltage supply circuit is provided to supply the voltage from the DC power source circuit or the DC-AC conversion circuit to the primary circuit to charge the capacitor of the primary circuit of each start circuit.
According to the present invention, the voltage supply circuit for supplying voltages to the capacitor of the primary circuit of a transformer comprising the start circuit, which is provided to each discharge lamp, is commonly provided to each start circuit. Thus, reduction in the cost and the size of the apparatus can be realized.